RE_FORMAT(7)					       BSD Miscellaneous Information Manual					      RE_FORMAT(7)

NAME

     re_format -- POSIX 1003.2 regular expressions

DESCRIPTION

     Regular expressions (``REs''), as defined in IEEE Std 1003.2 (``POSIX.2''), come in two forms: modern REs (roughly those of egrep(1); 1003.2
     calls these ``extended'' REs) and obsolete REs (roughly those of ed(1); 1003.2 ``basic'' REs).  Obsolete REs mostly exist for backward com-
     patibility in some old programs; they will be discussed at the end.  IEEE Std 1003.2 (``POSIX.2'') leaves some aspects of RE syntax and
     semantics open; `=' marks decisions on these aspects that may not be fully portable to other IEEE Std 1003.2 (``POSIX.2'') implementations.

     A (modern) RE is one= or more non-empty= branches, separated by '|'.  It matches anything that matches one of the branches.

     A branch is one= or more pieces, concatenated.  It matches a match for the first, followed by a match for the second, etc.

     A piece is an atom possibly followed by a single= '*', '+', '?', or bound.  An atom followed by '*' matches a sequence of 0 or more matches
     of the atom.  An atom followed by '+' matches a sequence of 1 or more matches of the atom.  An atom followed by '?' matches a sequence of 0
     or 1 matches of the atom.

     A bound is '{' followed by an unsigned decimal integer, possibly followed by ',' possibly followed by another unsigned decimal integer,
     always followed by '}'.  The integers must lie between 0 and RE_DUP_MAX (255=) inclusive, and if there are two of them, the first may not
     exceed the second.  An atom followed by a bound containing one integer i and no comma matches a sequence of exactly i matches of the atom.
     An atom followed by a bound containing one integer i and a comma matches a sequence of i or more matches of the atom.  An atom followed by a
     bound containing two integers i and j matches a sequence of i through j (inclusive) matches of the atom.

     An atom is a regular expression enclosed in '()' (matching a match for the regular expression), an empty set of '()' (matching the null
     string)=, a bracket expression (see below), '.' (matching any single character), '^' (matching the null string at the beginning of a line),
     '$' (matching the null string at the end of a line), a '' followed by one of the characters '^.[$()|*+?{' (matching that character taken as
     an ordinary character), a '' followed by any other character= (matching that character taken as an ordinary character, as if the '' had not
     been present=), or a single character with no other significance (matching that character).  A '{' followed by a character other than a digit
     is an ordinary character, not the beginning of a bound=.  It is illegal to end an RE with ''.

     A bracket expression is a list of characters enclosed in '[]'.  It normally matches any single character from the list (but see below).  If
     the list begins with '^', it matches any single character (but see below) not from the rest of the list.  If two characters in the list are
     separated by '-', this is shorthand for the full range of characters between those two (inclusive) in the collating sequence, e.g. '[0-9]' in
     ASCII matches any decimal digit.  It is illegal= for two ranges to share an endpoint, e.g. 'a-c-e'.  Ranges are very collating-sequence-
     dependent, and portable programs should avoid relying on them.

     To include a literal ']' in the list, make it the first character (following a possible '^').  To include a literal '-', make it the first or
     last character, or the second endpoint of a range.  To use a literal '-' as the first endpoint of a range, enclose it in '[.' and '.]' to
     make it a collating element (see below).  With the exception of these and some combinations using '[' (see next paragraphs), all other spe-
     cial characters, including '', lose their special significance within a bracket expression.

     Within a bracket expression, a collating element (a character, a multi-character sequence that collates as if it were a single character, or
     a collating-sequence name for either) enclosed in '[.' and '.]' stands for the sequence of characters of that collating element.  The
     sequence is a single element of the bracket expression's list.  A bracket expression containing a multi-character collating element can thus
     match more than one character, e.g. if the collating sequence includes a 'ch' collating element, then the RE '[[.ch.]]*c' matches the first
     five characters of 'chchcc'.

     Within a bracket expression, a collating element enclosed in '[=' and '=]' is an equivalence class, standing for the sequences of characters
     of all collating elements equivalent to that one, including itself.  (If there are no other equivalent collating elements, the treatment is
     as if the enclosing delimiters were '[.' and '.]'.)  For example, if 'x' and 'y' are the members of an equivalence class, then '[[=x=]]',
     '[[=y=]]', and '[xy]' are all synonymous.	An equivalence class may not= be an endpoint of a range.

     Within a bracket expression, the name of a character class enclosed in '[:' and ':]' stands for the list of all characters belonging to that
     class.  Standard character class names are:

	   alnum    digit    punct
	   alpha    graph    space
	   blank    lower    upper
	   cntrl    print    xdigit

     These stand for the character classes defined in ctype(3).  A locale may provide others.  A character class may not be used as an endpoint of
     a range.

     A bracketed expression like '[[:class:]]' can be used to match a single character that belongs to a character class.  The reverse, matching
     any character that does not belong to a specific class, the negation operator of bracket expressions may be used: '[^[:class:]]'.

     There are two special cases= of bracket expressions: the bracket expressions '[[:<:]]' and '[[:>:]]' match the null string at the beginning
     and end of a word respectively.  A word is defined as a sequence of word characters which is neither preceded nor followed by word charac-
     ters.  A word character is an alnum character (as defined by ctype(3)) or an underscore.  This is an extension, compatible with but not spec-
     ified by IEEE Std 1003.2 (``POSIX.2''), and should be used with caution in software intended to be portable to other systems.  The additional
     word delimiters '<' and '>' are provided to ease compatibility with traditional svr4(4) systems but are not portable and should be avoided.

     In the event that an RE could match more than one substring of a given string, the RE matches the one starting earliest in the string.  If
     the RE could match more than one substring starting at that point, it matches the longest.  Subexpressions also match the longest possible
     substrings, subject to the constraint that the whole match be as long as possible, with subexpressions starting earlier in the RE taking pri-
     ority over ones starting later.  Note that higher-level subexpressions thus take priority over their lower-level component subexpressions.

     Match lengths are measured in characters, not collating elements.	A null string is considered longer than no match at all.  For example,
     'bb*' matches the three middle characters of 'abbbc', '(wee|week)(knights|nights)' matches all ten characters of 'weeknights', when '(.*).*'
     is matched against 'abc' the parenthesized subexpression matches all three characters, and when '(a*)*' is matched against 'bc' both the
     whole RE and the parenthesized subexpression match the null string.

     If case-independent matching is specified, the effect is much as if all case distinctions had vanished from the alphabet.	When an alphabetic
     that exists in multiple cases appears as an ordinary character outside a bracket expression, it is effectively transformed into a bracket
     expression containing both cases, e.g. 'x' becomes '[xX]'.  When it appears inside a bracket expression, all case counterparts of it are
     added to the bracket expression, so that (e.g.)  '[x]' becomes '[xX]' and '[^x]' becomes '[^xX]'.

     No particular limit is imposed on the length of REs=.  Programs intended to be portable should not employ REs longer than 256 bytes, as an
     implementation can refuse to accept such REs and remain POSIX-compliant.

     Obsolete (``basic'') regular expressions differ in several respects.  '|' is an ordinary character and there is no equivalent for its func-
     tionality.  '+' and '?' are ordinary characters, and their functionality can be expressed using bounds ('{1,}' or '{0,1}' respectively).
     Also note that 'x+' in modern REs is equivalent to 'xx*'.	The delimiters for bounds are '{' and '}', with '{' and '}' by themselves ordi-
     nary characters.  The parentheses for nested subexpressions are '(' and ')', with '(' and ')' by themselves ordinary characters.  '^' is an
     ordinary character except at the beginning of the RE or= the beginning of a parenthesized subexpression, '$' is an ordinary character except
     at the end of the RE or= the end of a parenthesized subexpression, and '*' is an ordinary character if it appears at the beginning of the RE
     or the beginning of a parenthesized subexpression (after a possible leading '^').	Finally, there is one new type of atom, a back reference:
     '' followed by a non-zero decimal digit d matches the same sequence of characters matched by the dth parenthesized subexpression (numbering
     subexpressions by the positions of their opening parentheses, left to right), so that (e.g.)  '([bc])1' matches 'bb' or 'cc' but not 'bc'.

SEE ALSO

     regex(3)

     Regular Expression Notation, IEEE Std, 1003.2, section 2.8.

BUGS

     Having two kinds of REs is a botch.

     The current IEEE Std 1003.2 (``POSIX.2'') spec says that ')' is an ordinary character in the absence of an unmatched '('; this was an unin-
     tentional result of a wording error, and change is likely.  Avoid relying on it.

     Back references are a dreadful botch, posing major problems for efficient implementations.  They are also somewhat vaguely defined (does
     'a((b)*2)*d' match 'abbbd'?).  Avoid using them.

     IEEE Std 1003.2 (``POSIX.2'') specification of case-independent matching is vague.  The ``one case implies all cases'' definition given above
     is current consensus among implementors as to the right interpretation.

     The syntax for word boundaries is incredibly ugly.

BSD
								   June 30, 2014							       BSD